22a – Pathophysiology of Anesthesia: CV

Question 1

What does the medulla do?

Answer 1

• Controls sympathetic and parasympathetic nervous system output
• Receives feedback from various systems to maintain appropriate CO

Question 2

Sympathetic nervous system

Answer 2

• Release catecholamines (E, NE)
• Changes mainly affects arteries, arterioles and great veins

Question 3

Parasympathetic nervous system

Answer 3

• Release ACh
• Changes mainly affect cardiac rate and rhythm

Question 4

What is the ultra-short acting control?

Answer 4

• Basal vasomotor tone
• *not really affected by anesthesia
  o Can be disrupted by disease processes
• Autoregulation and endothelium derived factors

Question 5

Autoregulation

Answer 5

• Organs sense tissue O2 demands=increase/decrease blood flow to match
• Sense accumulation of K, H, Co2, adenosine, lactate

Question 6

What are the endothelial derived local vasodilators?

Answer 6

• NO
• PGI2

Question 7

What are the endothelial derived local vasoconstrictors?

Answer 7

• Endothelins
• Thomboxanes (A2)

Question 8

What is the short term control?

Answer 8

• Reflexes
  o Baroreceptor
  o Chemoreceptor
  o Bainbridge reflex
  o Frank-starling relationship
• *important for anesthesia
  o Blunted by inhaled and injectable anesthesia drugs

Question 9

Baroreceptor reflex

Answer 9

• In carotid sinus and aortic arch
• Sense changes in BP and result in changes to HR

Question 10

Chemoreceptor reflex

Answer 10

• Carotid and aortic bodies
  o Perfusion, CO2 and O2 blood levels and blood pH
• Feedback to brainstem to adjust ventilation and sympathetic activity

Question 11

Bainbridge reflex

Answer 11

• Stretch receptors in right atrium
• Sense increased pressure in R. atrium (increase venous return) and result in INCREASED HR

Question 12

Frank-Starling relationship

Answer 12

• Increased venous return stretches myocardium
• Leads to increased myocardial contractility and increased SV

Question 13

What is perfusion pressure?

Answer 13

• What provides adequate blood flow (perfusion) to tissues
  o MAP – pressure withing tissues
  o MAP>60mmHG=provides adequate tisse blood flow to major organ systems
• *changes in perfusion pressure leads to IMMEDIATE effects on organ function

Question 14

What is the cerebral perfusion pressure equation?

Answer 14

• MAP – intracranial pressure

Question 15

What is the renal perfusion pressure equation?

Answer 15

• MAP – glomerular capillary pressure

Question 16

What is the equation for the autoregulation zone? (brain and kidney perfusion)

Answer 16

• =range of MAP where organs maintain perfusion pressure to OPTIMIZE tissue perfusion
• *goal=maintain MAP>60mmHG
  o If lower than 60mmHG=outside autoregulatory zone, perfusion becomes dependant on systemic BP

Question 17

Coronary artery perfusion

Answer 17

• Myocardial perfusion occurs in DIASTOLE=depends on diastolic arterial pressure (DAP)
• *coronary artery perfusion=DAP – R. atrial pressure
• *goal=maintain MAP>40mmHg
  o If lower=reduced CAP leading to MYOCARDIAL ISCHEMIA

Question 18

Anesthesia effects: when a vasodilator

Answer 18

• Reduced systemic vascular resistance (SVR)
  o Inhale=dose dependent
  o Acepromazine=alpha1 adrenergic blockage
  o Meperidine/morphine IV=histamine release
  o Propofol and alfaxalone=preferential VENODILATION (after IV bolus)
• *effects are additive when used together
• *can lead to HYPOTENSION during anesthesia

Question 19

Anesthesia effects: when a vasoconstrictor

Answer 19

• Increase SVR
  o Alpha2 adernergic agonists=direct action on PERIPHERAL alpha-2 receptors
  o Ketamine and NO=sympathomimetic action that realeases endogenous catecholamines
• *can lead to HYPERTENSION
  o Can be seen in PREmedication, but reduced by inhaled anesthetics

Question 20

What are the drugs that have minimal vascular effects? (2)

Answer 20

• Opioids
• Benzodiazepines

Question 21

What do epidurals do to vasculature?

Answer 21

• Local anesthetic can travel to thoracic region of spinal cord=BLOCK sympathetic vasomotor nerves
  o *RESULT=reduced sympathetic control on vasomotor tone=vasodilation (decreased SVR)
  o Compounded by other vasodilatory anesthetic drugs
• **effects are TEMPORARY but extra care should be taken with large animals

Question 22

How can you avoid vasodilation from epidurals?

Answer 22

• *careful administration
• 1. Lowest volume required to provided adequate nerve blockade
• 2. SLOW injection speed=reduced injection pressure to slow forward spread

Question 23

What are some drugs that decrease myocardial contractility?

Answer 23

• Inhaled anesthetics, alfaxalone, propofol=decrease Ca availability
• Acepromazie (mild) and alpha2-adrenergic agonists
• Dose-dependent effects

Question 24

What are some states that reduce contractility?

Answer 24

• Hypoxemia
• Depleted catecholamines (sepsis)
• Acidosis (pH<7.0)
• Electrolyte imbalances (increase K, decreased Ca)
• Hypovolemia (Starling’s law)

Question 25

What are the values for prehypertension?

Answer 25

• SAP 140-150mmHg

Question 26

What are the values for hypertension?

Answer 26

• SAP>160-179mmHg

Question 27

What are the values for severe hypertension?

Answer 27

• SAP>180mmHg

Question 28

What can acute severe hypertension lead to?

Answer 28

• Decreased CO
• Edema
• Hemorrhage (brain/lungs)

Question 29

What does sustained hypertension result in?

Answer 29

• Myocardial remodelling
• Retinopathy or retinal detachment
• Encephalopathy
• Renal disease

Question 30

What is the treatment for hypertension?

Answer 30

• NEED TO ID underlying reason
  o *Sympathetic activation: light plane anesthesia, nociception, hypercapnia, hypoxemia
  o Diseases: chronic kidney disease, hyperadrenocorticism

Question 31

What are the values of hypotension (SAP, MAP, DAP)

Answer 31

• SAP<80mmHg
• MAP<60mmHg (large animals MAP<70mmHg, care more about skeletal muscle perfusion)
• DAP<40mmHg
• *more common

Question 32

What is significant hypotension?

Answer 32

• Severity
  o Mild/moderate: MAP<60mmHg
  o Severe: MAP<35-45mmHg
• Time frame
  o How quick?
  o How long?

Question 33

What are the sequelae of moderate, severe or prolonged hypotension (>5-15mins)

Answer 33

• Renal damage and poor urine production
• Hepatic and gastrointestinal damage (necrosis)
• Increased lactate concentration
• Myocardial ischemia, arrhythmias and cardia arrest

Question 34

What is HR and rhythm controlled by?

Answer 34

• Balance of autonomic nervous system
  o PS: vagus=release Ach=decreased HR
   Receptors: atria, conducting tissue
  o S: T1-T4 spinal nerves and cervical ganglia from cardiac nerves
   Release NE=increase HR
   B1 receptors: chronotropic, inotropic, dromotropic
   B2 rectors (atria): chronotropic

Question 35

What is bradycardia?

Answer 35

• *less than half of resting HR
  o Dogs: <50bpm
  o Cats: <90bpm

Question 36

What are bradyarrhythmias?

Answer 36

• Increase PS tone
  o Usually drug or disease induced

Question 37

What is tachycardia?

Answer 37

• Dogs >180bpm
• Cats >200bpm
• *species differences

Question 38

What are tachyarrhythmias?

Answer 38

• Increased sympathetic tone
  o Nociception
  o Anxiety
  o Hypercapnia
  o Hypoxemia
  o Hypovolemia
  o Drugs

Question 39

What does xylazine and ketamine do to heart rhythm?

Answer 39

• Stimulate alpha1 and beta1=tachyarrhythmias

Question 40

What do opioids and alpha2-adrenegic agonist do to hearth rhythm?

Answer 40

• Increase vagal (PS) tone=bradyarrhythmias

Question 41

What does acepromazine do to heart rhythm?

Answer 41

• Anti-arrhythmic=alpha1 adrenergic blockage

Question 42

What drugs have minimal to NO effect on heart rhythm?

Answer 42

• Inhalant anesthetics (until deep plane of anesthesia)
• Propofol, alfaxalone
• Benzodiazepine